In the synthesis of vitamin D.sub.2 and vitamin D.sub.3, it is common to use photochemicaI preparation processes. Thus, vitamin D.sub.3 is prepared commercially by photolysis of 7-dehydrocholesterol to form pre-vitamin D.sub.3 (P.sub.3) which is subsequently heated to form vitamin D.sub.3. The first stage of photolysis can however form by-products such as lumisterol (L.sub.3) and tachysterol (T.sub.3), from which vitamin D.sub.3 cannot be formed directly. Accordingly, such photolysis processes should minimize the formation of L.sub.3 and T.sub.3, or alternatively seek to convert the L.sub.3 and T.sub.3 formed back to P.sub.3, in order to maximize the yield of P.sub.3 and subsequently of vitamin D.sub.3.